The present invention relates generally to motor control and, more particularly, to a motor drive with synchronized timing.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Rotating motors are typically controlled by a motor drive that receives reference motor velocity and position signals and produces a torque signal that is applied to the motor. The torque signal is generally controlled using a pulse width modulated (PWM) technique. Adjustment of the torque signal based on changes to the reference velocity or position signals relative to measured feedback velocity and position signals ensures that the motor rotates at the reference velocity.
Some applications require precise motor control across multiple, synchronized motors. For example, an electronic line shaft may be employed in a printing application to move the paper or other material over rollers and through various stages of the printing process. For instance, an exemplary printing system may include a folding unit and a plurality of separate printing units where each printing unit includes a plurality or rollers, motors, motor controllers that move a web of paper along and apply different ink colors thereto. After printing, the printing units feed the folding unit that cuts the printed web into sheets and, as the label implies, folds the cut sheets to form sections of newspapers, books, magazines or the like. To ensure print quality, the various stages are synchronized. A lack of synchronicity between the stations results in misregistration between the colors and/or the cut sheets, leading to unacceptable product that may need to be scrapped.
Previous generations of printing technology employed a mechanical line shaft mechanically linked to the various printing units and the folding unit. Rotation of the line shaft by an electric motor activated rollers and other printing unit tools along the line to facilitate the printing process. In a mechanical line shaft system, factors such as play in the mechanical linkages, stretching of the paper web, and torsional flexing of the line shaft itself make it difficult to achieve and maintain synchronicity between the printing units, especially during periods of acceleration and deceleration of the printing system. It has been observed that when synchronicity is not maintained, product generated includes excessive flaws and is often unacceptable for intended use. Mechanical line shafts also have reduced flexibility in addressing print changes. Hence, where changes are required, down time may be excessive.
More modern printing systems, commonly referred to shaftless printing systems or electronic line shaft systems, employ a plurality of motors, motor controllers and associated rollers that form printing and folding units that are electronically synchronized, as opposed to mechanically synchronized. Lack of synchronicity in an electronic line shaft results in similar problems, such as color misregistration, evident in a mechanical line shaft system.
When operating a plurality motors synchronously in an automated system, several factors exist that may cause the position of the motors to deviate from each other even though they are all operating pursuant to a single reference velocity signal. For instance, motor inertia between motors at different stations is often non-uniform and can cause one motor to drift from the other motors.
One solution for reducing drift between motors in a printing system that includes a folding unit and a plurality of printing units has been to divide the printing units into unit groups (e.g., where a group includes printing units that are logically related—a group may include six printing units that apply color print to a single page where the print applied by the six units has to be precisely aligned), obtain a position signal from the folding unit and provide the position signal to each of the groups to be distributed via a high speed bus to units within the group. While this solution results in unit groups that each separately generate substantially aligned printed product, alignment is not exact as, even in the case of a high speed inter-group but, there is some propagation time between the group unit controllers and hence the controllers are slightly phase shifted. In addition, while the units in each separate group may generate substantially aligned printed product, the printed product from each group may be misaligned when received at the folder unit because of the propagation delay between the folding from which the position signal is obtained and the printing unit groups.
Therefore, it would be advantageous to have a more accurate configuration for synchronizing printing and folding units in a printing system. Similarly and more generally, it would be advantageous to have an extremely accurate synchronizing system for use in any electronic line shaft application.